Fausset's Bible Dictionary
(the English mean "unconquerable".) Unusually hard stones, as the diamond, which is a corruption of the word adamant; Hebrew shamir; Greek smiris . Probably the emery stone or the uncrystallized corundum (Ezekiel 3:9). Image for firmness in resisting the adversaries of the truth of God (Zechariah 7:12). Image of hard heartedness against the truth (Jeremiah 17:1). The stylus pointed with it engraves deeper than the common iron; with such a pen is Jerusalem's sin marked. Its absence from the high priest's breast-plate was because it could not be engraven upon; or perhaps it had not been introduced at that early time. (See .)
These files are public domain.
Fausset, Andrew R. Entry for 'Adamant'. Fausset's Bible Dictionary. https://www.studylight.org/dictionaries/eng/fbd/a/adamant.html. 1949.